Current methods for manufacturing optical fiber include the step of drawing the fiber from a preform. A preform is a cylindrical body having a diameter much greater than the diameter of the fiber, but having a scaled radial distribution of composition that is the same as that of the fiber that is drawn from it. Thus, the composition distribution of a fiber is determined by the steps in the fabrication of the preform. For example, typical optical fibers in current use include a core region of relatively high refractive index, surrounded by a cladding region of a different composition, having a refractive index that is lower than that of the core. To form such a fiber, the preform must have compositionally distinct regions corresponding, respectively, to the core and the cladding. To produce the regions of differing refractive index, dopants are incorporated in the glass.
In silica glass, for example, germanium is conventionally introduced as a dopant for raising the refractive index of the core glass. Phosphorus is also useful for raising the refractive index of the core glass. (In addition, phosphorus reduces glass viscosity, permitting preforms to be fabricated and fibers to be drawn at a reduced temperature.) On the other hand, fluorine is an effective dopant for lowering the refractive index of the cladding glass.
Three main processes are currently used to produce optical fibers. They are: Outside Vapor Phase Oxidation (OVPO), Modified Chemical Vapor Deposition (MCVD), and Vapor-phase Axial Deposition (VAD).
In OVPO, SiCl.sub.4 vapor is hydrolyzed in a torch flame, producing a stream of fine silica particles that are deposited along the outer surface of a rotating cylindrical mandrel. Dopant vapors can be mixed with the SiCl.sub.4 vapor. Thus, a core layer of relatively high refractive index is typically deposited first, using SiCl.sub.4 doped with GeCl.sub.4. A cladding layer having a composition adapted to yield a lower refractive index is deposited on the core layer. The mandrel is then removed, and the porous body is sintered to make the glass blank from which fibers are drawn.
In MCVD, chloride reagent gases such as SiCl.sub.4 and GeCl.sub.4 react homogeneously with oxygen inside a rotating glass tube, e.g., a fused quartz tube. The reaction produced by heating with an external torch produces silica particles that deposit thermophoretically on the inner wall of the tube to form a thin, porous layer. Each layer is vitrified by sintering it with heat from the same torch as it travels along the tube. The cladding material is deposited first, and then the core material. After the deposition steps are completed, the tube is collapsed to make the preform.
In VAD, as in OVPO, a stream of fine silica particles is deposited on a rotating mandrel. Here, however, particles are deposited on the end of the mandrel, and the porous preform is grown axially, rather than radially. The porous preform is pulled up through a furnace to sinter the deposited material.
When fibers for certain special applications are to be manufactured, it may be necessary to incorporate dopants other than those (e.g., GeO.sub.2, P.sub.2 O.sub.5, B.sub.2 O.sub.3) that are added primarily to alter the refractive index of the glass. For example, optical amplifiers, including both single-pass amplifiers and lasers, can be made by doping the cores of optical fibers with rare earth ions, such as the erbium ion Er.sup.3+, effectively making the fiber core into a laser medium. However, dopant cations, such as rare earth cations, are not as readily incorporated into the glass as are traditional dopants which are introduced as vapors or the liquid halides. Instead, special methods need to be used to dope the glass.
For example, optical fiber preforms made by the MCVD method may be doped by volatilizing and entraining dopant material, e.g., aluminum chloride, in a heated carrier gas such as helium, and adding the resulting gaseous mixture to the reactant gases within the fused quartz tube. However, although this approach is useful, it tends to produce a dopant concentration gradient along the axis of the tube, which is difficult to suppress. In addition, when making a rare-earth-doped optical amplifier, it is desirable to add mixtures of dopants, such as erbium together with aluminum or ytterbium, in order to optimize the optical properties of the amplifier. Because of differences in the volatilities of different dopant materials, the relative proportions of co-dopants present in the glass are difficult to control, and they tend to vary along the axis of the quartz tube. An additional problem is involved when dopant metals are introduced to the fused quartz tube as chloride vapors, and are to be reacted with oxygen to form metal oxides and free chlorine gas. The thermodynamic equilibrium for this reaction is unfavorable in many cases, and therefore it is difficult to incorporate many desirable dopant species by this method. On the other hand, many metal halides are refractory, and therefore cannot be readily introduced directly in the vapor phase.
As another example, desired active ions can be incorporated in the porous soot boules created by the OVPO or VAD processes by "solution doping." That is, the boule is immersed in a solution containing small amounts of dopant material, which is absorbed into the pores of the boule. The boule is then dried, dehydrated in chlorine at about 1000.degree. C. to remove OH, and sintered to a solid preform. (MCVD preforms can also be doped in this manner. An MCVD preform is prepared for solution doping by only partially sintering a deposited layer, prior to immersion.)
This method suffers the disadvantage that it is prone to the formation of clusters and microcrystallites of dopant material. That is, because dopant material is not distributed evenly, but rather is concentrated in the pores of the preform, clusters or microcrystallites of dopant material tend to form both before sintering and during the steps when the glass body is sintered and collapsed. Although microcrystals of refractory oxides such as Er.sub.2 O.sub.3, for example, may melt or dissolve during the collapse phase, diffusion is unlikely to achieve homogeneity. Consequently, regions of high Er.sub.2 O.sub.3 concentration tend to remain as clusters in the preform.
Microcrystallites are undesirable because, inter alia, they can scatter light. Clusters are undesirable because they absorb light. For example, in three-level laser systems such as Er.sup.3+, excited state absorption (ESA) reduces the laser efficiency. ESA is exacerbated if Er.sup.3+ ions are in close physical proximity to one another, as in a cluster.
Another disadvantage of solution doping is that when the solution evaporates, it leaves behind a residue containing the starting species, e.g., ErCl.sub.3.6H.sub.2 O (or erbium oxychloride, to which the chloride is converted upon heating). However, during dehydration in chlorine at high temperature, erbium may be volatilized from the surface (i.e., near the inner surface of the deposited layer within the fused quartz tube), creating a dip in concentration near the inner surface of the porous layer. Thus, although the process is effective for doping the glass because the erbium becomes trapped in the pores, volatilization creates a dip in the dopant concentration profile at the center of the final fiber. This is undesirable because both the signal and the pump light are most intense at the center of the fiber. For the amplifier, e.g., laser, to be as efficient as possible, it is important to assure that only pump light of high intensity will interact with the dopant. This is particularly true because below a certain intensity, dopant ions will only absorb, rather than emit, optical energy, thus decreasing the amplifier's gain.
As an alternative to solution doping, porous boules created by VAD can be doped by exposure to dopant vapor, for example, to erbium chloride vapor, in a heated chamber, e.g., a chamber used for sintering. However, it is difficult to achieve high dopant concentrations by this method, and clustering, phase separation, and devitrification are potential problems.
Thus, practitioners of the art of manufacturing specially doped optical fibers have so far been unsuccessful in the search for a doping method that provides good reproducibility from preform to preform, good spatial uniformity of composition and avoids the problem of aggregation of dopant material into clusters and microcrystallites at rare earth dopant concentrations greather than 500 ppm.